Variable air volume systems are commonly installed in new commercial and industrial buildings. A primary reason for the popularity of variable air volume systems is their energy efficiency. However, for a variable volume system to operate at its most efficient level, it is necessary to control the supply fan as a function of the system static pressure, to volumetrically synchronize the supply fan and return fan (if one is used), to control the building pressure, and to control the minimum outside air provided to the building to meet ventilating code requirements.
In modern variable volume systems, the control of the supply fan as a function of system static pressure is typically accomplished by using a static pressure sensor located toward the terminus of the air supply system, usually approximately two-thirds of the distance into the system or at a representative location at which monitoring is preferred. Commercially available static pressure sensors provide an output signal that is then processed into an electronic or pneumatic signal which is proportional to the static pressure sensed. The signal proportional to pressure is then used to control the volume of the supply fan so as to maintain the static system pressure reasonably constant despite changes in system demand.
For systems utilizing a return fan as well as a supply fan, it has been typical to use flow sensors on the output of the supply fan and on the input or output of the return fan, allowing the flow rates of the two fans to be compared, and using the comparison signal to control the return fan volume. For example, the volumetric output of the supply and return fans may be controlled by utilizing motorized inlet dampers on the two fans. The supply and return fans are controlled so that a pre-selected flow rate differential between the two fans is maintained, with the return fan having a lower flow rate than the supply fan. The flow differential between the two fans corresponds to the flow of air exhausted from the building to satisfy code requirements. The output of the return fan is directed back to the input of the supply fans, and the difference in flow rate between the two fans is made up by connecting an outside air duct to the input of the supply fan, thereby allowing outside air to flow into the supply fan at a rate equal to the differential between the flows through the supply fan and return fan. The mixture of recirculated and outside air provided to the supply fan may be tempered--heated or cooled--at the input or the output of the supply fan.
Variable air volume systems utilizing a return fan as well as a supply fan provide reasonably good control of the ventilation requirements and pressurization of the building and relative control of the minimum outside air supplied to the building. However, even the best of such systems have certain limitations. Most fundamentally, the minimum outside air supplied to the building may not be precisely controlled because of variations in the flows through the supply and return fans and also because of infiltration of outside air into the building through vents, cracks, etc. In addition, an exhaust opening is typically provided near the outlet of the return fan to allow partial or complete exhaust of the return air from the building under some operating conditions. However, even when the exhaust opening is closed, some air may leak through the damper louver which closes the opening. Since the outside air infiltrating the building is umtempered, generally being either too warm or too cool, the introduction of excess outside air results in energy inefficiencies. The infiltrating air drawn into the building is also initially unfiltered.
Where there is a substantial temperature difference between the outside air and the air being recirculated within the ventilation system, the air drawn in at the intake may not mix well with the recirculated tempered air, allowing stratification of hot and cold air within the supply ducts. The stratification of the air may persist even through the supply fan. As a result, moisture condensation may occur in the supply ducts, and the separation of warm and cool air within the ducts may lead to distribution of air at some parts of the system which is too warm while other parts receive air which is too cool. The present supply fan-return fan variable air volume systems also utilize relatively complex and expensive control components, including velocity sensors for both the supply and return fan, a static pressure sensor in the supply duct, motorized inlet dampers for both the supply and return fans, motor starters for both fans, and an air flow instrumentation control center for processing the signals from the velocity sensors and the pressure sensor and providing control signals to the motorized inlet dampers on the supply and return fans. Since the supply and return fans are operated at a constant speed, with the inlet dampers to the two fans being periodically adjusted to control the volume output of each fan, the power consumed by the fans may often be in excess of that necessary to drive the volume of air that the system requires.